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Server functionality for HBase

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/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.hadoop.hbase.master.balancer;

import org.apache.hadoop.conf.Configuration;
import org.apache.yetus.audience.InterfaceAudience;

/**
 * Compute the cost of a potential cluster configuration based upon how evenly distributed tables
 * are.
 */
@InterfaceAudience.Private
class TableSkewCostFunction extends CostFunction {

  private static final String TABLE_SKEW_COST_KEY =
    "hbase.master.balancer.stochastic.tableSkewCost";
  private static final float DEFAULT_TABLE_SKEW_COST = 35;
  DoubleArrayCost[] costsPerTable;

  TableSkewCostFunction(Configuration conf) {
    this.setMultiplier(conf.getFloat(TABLE_SKEW_COST_KEY, DEFAULT_TABLE_SKEW_COST));
  }

  @Override
  void prepare(BalancerClusterState cluster) {
    super.prepare(cluster);
    costsPerTable = new DoubleArrayCost[cluster.numTables];
    for (int tableIdx = 0; tableIdx < cluster.numTables; tableIdx++) {
      costsPerTable[tableIdx] = new DoubleArrayCost();
      costsPerTable[tableIdx].prepare(cluster.numServers);
      final int tableIndex = tableIdx;
      costsPerTable[tableIdx].applyCostsChange(costs -> {
        // Keep a cached deep copy for change-only recomputation
        for (int i = 0; i < cluster.numServers; i++) {
          costs[i] = cluster.numRegionsPerServerPerTable[tableIndex][i];
        }
      });
    }
  }

  @Override
  protected void regionMoved(int region, int oldServer, int newServer) {
    int tableIdx = cluster.regionIndexToTableIndex[region];
    costsPerTable[tableIdx].applyCostsChange(costs -> {
      costs[oldServer] = cluster.numRegionsPerServerPerTable[tableIdx][oldServer];
      costs[newServer] = cluster.numRegionsPerServerPerTable[tableIdx][newServer];
    });
  }

  @Override
  protected double cost() {
    double cost = 0;
    for (int tableIdx = 0; tableIdx < cluster.numTables; tableIdx++) {
      cost += costsPerTable[tableIdx].cost();
    }
    return cost;
  }
}